Increasing evidence indicates that production of new neurons in the adult brain is important for behavior, brain repair, and response to therapies. Aging is associated with a profound decline in production of new neurons and decreased neurogenesis may be related to the impaired functioning of the aging brain. This decrease may be due to several factors including changes in the division of stem and progenitor cells, survival of stem cells or their progeny and altered fate of the stem cell progeny. To understand the mechanism of age-related decline in adult neurogenesis it is necessary to identify the changes in the main parameters of the division/ differentiation cascade in the aging brain. We developed an experimental platform that combines reporter mouse lines, lineage tracing techniques, precise quantitation and computational modeling to determine changes in stem cells division and fate. We used this approach to determine the neurogenic targets of antidepressant treatments and of brain injury. Furthermore, we will combine genetic stem cell analysis with a new technology of unparalleled sensitivity and accuracy, Multi-Isotope Imaging Mass Spectroscopy (MIMS), for the quantitative analysis of stem cells turnover and fate in the aging brain. Since our reporter mouse lines mark stem and progenitor cells in several non-neuronal tissues, we are in a position to analyze the same individual animals for parallel changes in several stem cell compartments. The main goal of this proposal is to apply stringent quantitative methods to the problem of stem cell lineage and to determine the changes in the turnover rates and the lineages of stem cells in response to aging, antidepressant therapies and injury. In Aim 1 we will use reporter lines, single- and double-label pulse chase analysis, computational modeling and MIMS to determine age-related changes in the parameters of the division/differentiation cascade of neural stem and progenitor cells in the dentate gyrus of the hippocampus. In Aim 2 we will use multi-allelic reporter lines, Cre-based lineage tracing, and MIMS to determine age-related changes in the fate of stem cells. In Aim 3 we will apply genetic fate mapping and MIMS to determine the age-associated changes in the parameters of the division/differentiation cascade and in the fate of stem cells in response to antidepressant treatments of different modalities (fluoxetine, electroconvulsive shock and exercise) and to injury that induces depressive behavior (ablation of dopaminergic neurons). Finally, in Aim 4, we will use genetic fate mapping and MIMS to determine changes in the turnover and fate of stem/progenitor cells in a range of non-neuronal tissues that are marked by reporter gene expression in our multi-allelic reporter lines. Together, our results and genetic tools will provide a framework for studying division and differentiation of stem cells and tissue turnover as an integrated quantitative endeavor.